In an effort to study the role of Borrelia burgdorferi proteins whose expression respond to changes in culture temperature, strain Sh-2-82 spirochetes were serially passaged 16 times in vitro. Following serial passage, the protein and plasmid DNA profiles of the passage 16 variant were compared with that of the low-passage (passage 3) parental culture to assess the loss of protein expression and plasmid DNAs. The expression of 16 antigens in the parental culture was found to be selectively induced at the higher incubation temperature of 34?C and not at the lower temperature of 24?C. A majority of the 16 parental antigens were subsequently lost in the higher passage variant. The loss of expression of these antigens in the higher passage variant also correlated with the loss of infectivity for the C3H/HeN mouse. Two of the 16 antigens were identified as the lipoproteins OspC, an antigen previously known to be temperature-inducible (Stevenson, B. T. G. Schwan, and P. A. Rosa, Infect. Immun. 63:4535-4539, 1995) and P35. The loss of antigen expression could not be correlated, however, with a detectable loss of plasmid sequences; specifically, the plasmids carrying the ospC and P35 genes were both found to be retained in the passage 16 variant. The temperature-inducible expression of OspC and P35 in the parental Sh-2-82 spirochetes and their minimal levels of expression in the passage 16 variant, both correlated with the steady state levels of the corresponding mRNAs. These data suggest that temperature-inducible antigens are essential for infectivity and that the regulated expression of at least two of these antigens, OspC and P35, is controlled at the transcriptional level. FUNDING Base PUBLICATIONS None